<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>vue-监听</title>
    <script src="https://unpkg.com/vue/dist/vue.js"></script>
</head>
<body>
<div id="app">
    <p style="font-size:25px;">计数器: {{ counter }}</p>
    <button @click="counter++" style="color: yellow;background-color: gray">点我</button>
</div>
<script>
    var vm=new Vue({
        el:'#app',
        data:{
            counter: 1
        }
    })
    vm.$watch('counter', function (nval, oval) {
        alert('计数器值的变化 :' + oval + ' 变为 ' + nval + '!');
    });

</script>
</body>
</html>